All electromagnetic types of electromechanical transducers for converting energy between a mechanical form and an electrical form operate on the same basic principles utilizing a time change of magnetic flux through a coil of wire. The differences between them lie in their practicality and relative efficiency.
In order to optimize efficiency and practicality, a variety of different parameters must be considered and optimized. For example, in a linearly reciprocating machine, such as a linear alternator, it is desirable to minimize the mass which must be driven in reciprocation in order to minimize the momentum which must be overcome by the driving forces. Similarly, it is desirable to minimize the total mass of an electrical alternator in order to provide the maximum ratio of energy output to alternator weight.
One way to help accomplish this is to maximize the electric current at which the machine can operate before the transformer iron, which is used to provide a high permeability flux path, begins to saturate. The flux causing saturation arises from two components, one from the source of magnetic flux, such as a permanent magnet and the other from the current induced in the armature winding. The latter source, referred to as the armature reaction flux, is proportional to the armature current divided by air gap distance. Thus, reducing the armature reaction flux by increasing the number of working gaps permits a higher operating current before saturation.
A goal in the design of a linear alternator intended to be driven by a Stirling engine is that it be axially symmetrical so that it can be rotated or spun about its central axis in order to permit the advantages of spin lubrication of a free-piston Stirling engine as described in U.S. Pat. No. 4,330,993.
A variety of alternator designs have been suggested in the prior art but different ones of them have inherent weaknesses. Some designs require the use of two different magnets for obtaining flux reversal, one for each oppositely directed flux polarity. Others utilize multiple magnets which have their poles interfacing in a manner to oppose or buck each other. These cause unnecessary complexity compared to the present invention.
Other designs are electrically inefficient, in that the magnet flux is sufficient to nearly saturate the iron, leaving only a small margin for armature reaction. Optimum design, approached by present invention, is to have equal armature reaction flux and magnet flux at iron saturation. This maximizes the power to weight ratio.
Further, it is well known that the relative motion of a magnet with respect to iron core material creates a substantial magnetic spring force tending to move the magnet toward a position of equilibrium. In some devices the equilibrium position is intermediate the opposite boundaries of the reciprocation path. However, if a linear alternator is driven by a free-piston Stirling engine, it is desirable that the linear alternator have two positions of equilibrium relatively near the opposite ends of the reciprocation path to facilitate starting the engine.
It is desirable that each turn of the windings be of minimum length and that the windings be wound compactly.
In some designs, one of the relatively reciprocating parts reciprocates into a space which could otherwise be occupied by winding conductors. This increases the weight by requiring more iron to position the windings away from moving magnets.
Still other designs, such as that prior art illustrated in FIG. 1, generate waveforms with undesirable characteristics as described below.